Formal letter structure is being determined by the number of so called conventions one should adhere to. It is essential that structure of the formal letter follows these conventions, because they have to reflect official character. Nevertheless overall formal letter structure must be concise, in order to deliver clear message to the reader. Therefore use of colloquialisms, often present in oral language, should be avoided.
In general formal letter structure can be as follows.
Detailed description and guidelines for addressing and salutations can be found on this link .
The first paragraph should be short and state the purpose of the letter- to make an enquiry, complain, request something, etc.
The paragraph or paragraphs in the middle of the letter should contain the relevant information behind the writing of the letter. Most letters in English are not very long, so keep the information to the essentials and concentrate on organising it in a clear and logical manner rather than expanding too much.
The last paragraph of a formal letter should state what action you expect the recipient to take- to refund, send you information, etc.
Ending, as described in the article on this page.